Method for pressing a plastic, which cures by means of a reaction, into a mould cavity, a pressing auxiliary in pill form to be used in this method and a holder composed of such material

ABSTRACT

Method for pressing a plastic (6), which cures by means of a reaction, from a displacement chamber (4) via at least one injection channel (3) and a gate (2) to a mould cavity (1), said method at least comprising the following steps: pressurising the plastic (6) with an amount of a pressing auxiliary (7) in the displacement chamber (4); pressing the plastic (6) into the mould cavity (1); and allowing the plastic to cure under pressure in the mould cavity, the pressing auxiliary (7) being plastically deformable under the conditions of pressing and the amount thereof being sufficient to be pressed to the vicinity of the gate (2) of the mould cavity (1). Preferably the pressing auxiliary (7) is a thermoplastic.

The present invention firstly relates to a method for pressing aplastic, which cures by means of a reaction, from a displacement chambervia at least one injection channel and a gate into a mould cavity, saidmethod at least comprising the following steps:

pressurising the plastic with an amount of a pressing auxiliary in thedisplacement chamber;

pressing the plastic into the mould cavity; and

allowing the plastic to cure under pressure in the mould cavity,

the amount of pressure auxiliary being sufficient to be pressed to thevicinity of the gate of the mould cavity.

The term plastic which cures by means of a reaction is used to designateplastics such as, for example, thermosetting plastics andphotopolymerisable plastics, multicomponent plastics and the like. Theseare plastics which cure by supplying heat or light or after mixing witha chemical compound. Examples of such plastics are generally known tothose skilled in the art. The plastics which cure by means of moistureor air can also be mentioned in this context. Usually howeverthermosetting plastics are used.

The abovementioned method is generally known and is used for theproduction of diverse mouldings from plastics which cure by means of areaction. The encapsulation of integrated circuits in thermosettingplastics, and the production of car components from such plastics, can,for example, be considered in this context.

In this respect reference is made to U.S. Pat. No. 3,911,075. In saidspecification a method is described, in which method a firstthermosetting polymeric material is used as the plastic which cures bymeans of a reaction. Further a cheap second thermosetting polymericmaterial is used as the pressing auxiliary. By pressing the twopolymeric materials together from a displacement chamber, through aninjection channel and a gate into a mould cavity, the passages up to thegate are filled with the relatively cheap thermosetting polymericmaterial.

Pressing of plastics which cure by means of a reaction into a mouldcavity is, as a consequence of the properties of these plastics, usuallya batchwise method, because the curing reaction is frequently exothermicand is a reaction which goes to completion. For example, thermosettingplastics are plastics which melt by supplying heat thereto and then cureby means of an exothermic reaction. As a consequence thereof,thermosetting plastics have a relatively short storage time and mustalso be stored cool. Comparable storage problems arise in the case ofplastics which cure by means of light, air or moisture. Althoughmulticomponent plastics, which cure by mixing the various componentswith one another, have fewer storage problems, the curing reactionusually goes to completion after mixing.

Plastics which cure by means of a reaction are usually placed in adetermined amount, for example in pill form, together with a, e.g.thermosetting, pressing auxiliary, in a displacement chamber which isconnected via injection channels to the gates of the mould cavities. Theplastic and pressing auxiliary are then pressed with the aid of aplunger or the like through the injection channels and the gates intothe mould cavities. With this method it can be advantageous, for examplein the case of thermosetting plastics, to heat the plunger or thedisplacement chamber. The plastic itself can also be preheated.

Once the mould cavities have been filled with the plastic, said cavitiesmust be kept under pressure during curing of the plastic, in order toprevent bubble formation as a result of any entrapped air or moisturewhich may be present. This is done by pressurising the pressingauxiliary.

By using a pressing auxiliary a number of significant disadvantages ofusing only the plastic which cures by a reaction are overcome. Arelatively large amount of cured plastic, which remained behind in saidchannels and in the displacement chamber after curing, is saved.

The materials used as pressing auxiliaries are in the state of the artlimited to thermosetting plastics (e.g. U.S. Pat No. 3,911,075). Theseplastics have proven to be quite useful although they still possess anumber of important disadvantages. Thermosetting materials usually havean exceptionally good adhesive strength to the mould (mould cavity, theinjection channels and the displacement chamber) after curing thereof.To date, in order to lower this adhesive strength, release agents, suchas release wax and the like are added to the plastic, which additivesare not advantageous if an article, such as, for example, an integratedcircuit, has to be coated with the plastic, as the release agent willinherently deteriorate the adhesion to said integrated circuit.

Thermosetting plastics will always react to completion and are thereforenot re-usable, which is a problem as a consequence of the fact that theyare rather expensive and environmentally unfriendly plastics.

Further it is difficult to prevent mixing when using a pressingauxiliary consisting of thermosetting plastic during the filling of amould cavity with another thermosetting plastic. Thermosetting plasticscompletely melt with relatively small viscosity differences, beforecuring.

The object of the present invention is to provide a solution for theabovementioned disadvantages and to this end it is characterised in thatas the pressing auxiliary a material is used which is plasticallydeformable under the conditions of pressing and reversibly coagulates orhardens after pressing.

Surprisingly, it has been found possible, while pressing two differentmaterials at the same time, nevertheless completely to fill the mouldcavity with the plastic which cures by means of a reaction and to fillthe remaining space, that is to say the remaining space in thedisplacement chamber and the injection channels, essentially to thevicinity of the gate, with the pressing auxiliary, essentially no mixingtaking place. Further the method according to the invention has theadvantage that the pressing auxiliary is re-usable and can be chosenfrom environmentally friendly materials.

Preferably under the conditions of pressing, the pressing auxiliary hasa higher viscosity than the plastic which cures by means of a reaction.

Advantageously, the pressing auxiliary is a thermoplastic. Suitablethermoplastics are, for example, polyesters, polyethylene, polypropyleneetc. The advantage of thermoplastics is that these can be re-used andthat these, moreover, have a relatively low adhesive strength to themould (mould cavity, injection channel, etc.). Moreover, it would bepossible to add release agents thereto, since the addition of releaseagents to the pressing auxiliary has no influence on the moulding to beproduced. Thermoplastics are reversibly softened by a temperature rise.

The pressing auxiliary according to the invention can be pressed in thedisplacement chamber in a very large number of different ways, dependingon the plastic which cures by means of a reaction which is used and theform in which it is used. If, for example, a plastic in pill form isused, it is preferred also to press the pressing auxiliary in pill formin the displacement chamber with the thermosetting plastic.

However, it has also been found to be possible to place a plastic, whichcures by means of a reaction, in a holder beforehand and optionallyunder vacuum. In this case it is advantageous to produce the holder fromthe pressing auxiliary according to the invention, which holder can beprocessed after filling with the plastic. The holder may locally beprovided with thickened sections, if necessary, in order to be able toprovide the required amount of pressing auxiliary which is needed tofill the volume between the mould cavity and the displacement chamber.Of course, it is also possible to introduce the pressing auxiliary inpill form into the displacement chamber alongside the holder, or toincorporate supplementary pressing auxiliary in the holder alongside theplastic which cures by means of a reaction.

Reference is made to DE-A-1 812 217 which describes a method forinjection moulding of hardenable liquid masses for form-press andencapsulation applications. The problem for which a solution is providedresides in the fact that the filling of the hardenable liquids in thedisplacement chamber substantially reduces the speed of the injectionmoulding method, and that the risk exists of hardening of the liquidalready in the displacement chamber or the injection channels prior tocomplete filling of the mould cavity. The solution is to use a packagecontaining the liquid mass. Said package may be produced from aluminiumfoil or Mylar-Polyester foil. The use of a pressing auxiliary is howevernot disclosed.

The invention further provides a pressing auxiliary in pill form, madeof a material which is plastically deformable under conditions ofpressing and reversibly coagulates or hardens after pressing, for use inthe method according to the invention.

Finally, the invention provides a holder made of a material which isplastically deformable under conditions of pressing and reversiblycoagulates or hardens after pressing, filled with or obviously intendedto be filled with a plastic which cures by means of a reaction, for usein the method according to the invention.

The invention will be illustrated in more detail below with reference tothe appended drawing, in which:

FIGS. 1a-d show a diagrammatic representation of various sub-steps Of anillustrative embodiment of the method according to the invention;

FIG. 2 shows an example of a plastic, which cures by means of areaction, in pill form and the pressing auxiliary in pill form;

FIG. 3 shows a bowl-shaped holder made of pressing auxiliary which has athickened base and is filled with a plastic which cures by means of areaction;

FIG. 4 shows a bag-shaped holder which is made of pressing auxiliaryaccording to the invention and is filled with a plastic which cures bymeans of a reaction and pressing auxiliary granules according to theinvention; and

FIG. 5 shows an element which consists of two separate plasticcomponents and an amount of pressing auxiliary.

In FIG. 1, 1 indicates a mould cavity, 2 a gate, 3 an injection channeland 4 a displacement chamber. I and II show, diagrammatically, two mouldhalves. A plunger 5, shown diagrammatically, and a pill 6, in this casecomposed of a thermosetting plastic, are located in the displacementchamber 4. By way of example, a pressing auxiliary consisting of apolyester, in this case polyethylene terephthalate, is incorporated herebetween the pill and the plunger. The manner in which the pressingauxiliary is present in the displacement chamber 4 is not very criticalin this case. It would also be possible to supply several small pills.It will be obvious that the displacement chamber 4 can be connected in agenerally known manner with several mould cavities 1.

If the mould halves I and II are heated and the plunger 5 is pressed inthe direction of the thermosetting plastic pill 6, the pill will melt atits top and bottom surfaces and will be pressed via the injectionchannel 3 and the gate 2 into the mould cavity 1, as shown stepwise inFIGS. 1b and c.

As a result of the rise in temperature, which originates both from thetwo mould halves I and II and from the heat of reaction of the curingreaction of the thermosetting plastic, the polyethylene terephthalatepill is softened and the latter can be pressed behind the thermosettingplastic into the injection channel 3, as is shown in FIG. 1d.

It is within the capacity of the average person skilled in the art tochoose the amount of polyethylene terephthalate in such a way that thisis just sufficient to fill the space in the injection channel 3 and theremaining space in the displacement chamber 4. The interface between thethermosetting plastic 6 and the pressing auxiliary 7 is, of course,preferably at the gate 2, or can also be somewhat upstream of this inthe injection channel 3 or also somewhat downstream of this in the mouldcavity 1. The particular conditions depend on the application concerned.

In the state shown in FIG. 1d, the mould cavity 1, which is filled witha thermosetting plastic 6, can be kept under pressure by the presence ofthe polyethylene terephthalate 7, so that bubble formation in themoulding is prevented. After curing and opening the mould I, II, themoulding can be broken off at the location of the gate 2 and the surplusresidual material substantially consisting of polyethylene terephtalatecan be re-used.

The use of the pressing auxiliary according to the invention also hasthe advantage that the unusable residual parts of the injection channels3 and the displacement chamber 4 can easily be detached from the mouldparts I, II.

It will be obvious that the method according to the invention can alsobe used in other fields where similar problems arise.

FIG. 2 shows a perspective view of a plastic, which cures by means of areaction, in the form of a pill 9, as well as a pressing auxiliary 8 inpill form made of a material which is plastically deformable underpressing conditions and reversibly coagulates or hardens after pressing.

FIG. 3 shows a diagrammatic cross-section of a bowl-shaped holder 10which comprises a lid 11 and a thickened base 12 and is filled with aplastic 13 which cures by means of a reaction. The holder 10 can, forexample, be placed under vacuum in order to prevent the intake of air ormoisture. A filled holder 10 of this type can be placed in its entiretyin a displacement chamber in order to be emptied by pressing, withbreakage of the lid, into a mould cavity. When the holder 10 is empty,the holder material itself will also be pressed in order to fill thespace between the mould cavity and the displacement chamber and thus tokeep the mould cavity under pressure.

FIG. 4 shows a bag-shaped holder 13, which preferably consists ofpressing auxiliary according to the invention and is filled with plastic14, which cures by means of a reaction, and granules 15 of pressingauxiliary according to the invention.

Finally, FIG. 5 shows, diagrammatically, a two-component plastic elementwhich has two compartments 16 and 17, which contain two components aswell as an amount of pressing auxiliary 18 according to the invention.The two compartments 16 and 17 are separated by a breakable partition19. When pressure is applied in the displacement chamber on the side ofthe pressing auxiliary 18, the partition 19 will be broken and the twocomponents will come into contact with one another and be pressed intothe mould cavity, where the curing reaction takes place.

I claim:
 1. A method of pressing a plastic, which cures by means of areaction, from a displacement chamber through an injection channel and agate into a mold cavity and comprising the steps of positioning aplastic to be pressed and a pressing auxiliary in a displacementchamber, pressurizing the pressing auxiliary and the plastic andpressing the plastic into the mold cavity, allowing the plastic in themold cavity to cure while under pressure, the quantity of the pressingauxiliary in the displacement chamber being sufficient to be pressed tothe vicinity of the gate of the mold cavity, the pressing auxiliarycomprising a material which is plastically deformable under conditionsof the pressing and reversibly coagulates or hardens after pressing. 2.Method according to claim 1, wherein under the conditions of pressing,the pressing auxiliary has a higher viscosity than the plastic whichcures by means of a reaction.
 3. Method according to claim 1, whereinsaid pressing auxiliary is a thermoplastic.
 4. Method according to claim1, wherein said pressing auxiliary is pressed in pill form in thedisplacement chamber.
 5. Method according to claim 1, wherein saidpressing auxiliary comprises a holder for the plastic which cures bymeans of a reaction, such that the holder can be processed after fillingwith said plastic.
 6. A pressing auxiliary in pill form, made of amaterial which is being plastically deformable under conditions ofpressing and reversibly coagulates or hardens after pressing for use ina method of pressing a plastic, which cures by means of a reaction, froma displacement chamber through an injection channel and a gate into amold cavity and comprises the steps of positioning a plastic to bepressed and a pressing auxiliary in a displacement chamber, pressurizingthe pressing auxiliary and the plastic and pressing the plastic into themold cavity, allowing the plastic in the mold cavity to cure while underpressure, the quantity of the pressing auxiliary in the displacementchamber being sufficient to be pressed to the vicinity of the gate ofthe mold cavity.
 7. A holder made of a pressing auxiliary material beingplastically deformable under conditions of pressing and reversiblycoagulates or hardens after pressing, filled with a plastic which curesby means of a reaction for use in a method of pressing a plastic, whichcures by means of a reaction, from a displacement chamber through aninjection channel and a gate into a mold cavity and comprises the stepsof positioning a plastic to be pressed and a pressing auxiliary in adisplacement chamber, pressurizing the pressing auxiliary and theplastic and pressing the plastic into the mold cavity, allowing theplastic in the mold cavity to cure while under pressure, the quantity ofthe pressing auxiliary in the displacement chamber being sufficient tobe pressed to the vicinity of the gate of the mold cavity.